The present invention relates to a spindle motor driving apparatus for a disc device, and more particularly, to improvements in the starting rising characteristics of a spindle motor.
FIG. 1 shows a block diagram of a prior-art driver for driving a motor of a disc device. In FIG. 1, numeral 1 designates a spindle motor (hereinbelow, termed "motor"), which drives one or a plurality of magnetic discs, not shown, numeral 2 designates a spindle motor driving circuit (hereinbelow termed, "a driving circuit"), which drives the motor 1 and includes a pair of upper and lower transistors Q.sub.1 and Q.sub.2 to drive one phase of the motor 1 and a a motor current detecting resistor R which is used to control current for turning the motor. Numeral 3 designates a rotary angle position sensor necessary to decide the direction of driving current to the motor 1 and to detect a driving current flowing zone and motor turning speed. The rotary angle position sensor may for example, employ a Hall element. The output of the sensor 3 is input to a turning speed comparator 4, which compares the output of the sensor 3 with the output of a target turning speed generator 5 to control the turning speed of the motor 1 constantly. Numeral 6 designates a current limiter for protecting the transistors of the driving circuit 2, and numeral 7 designates a d.c. power supply, which supplies current to drive the motor 1. The above-described prior-art is a general constant motor turning speed control circuit, and the function of the current limiter 6 will be further described.
The d.c. resistance of one-phase shunt winding of the motor 1 having, for example, approx. 1/8 horsepower is approx. 0.6 ohm. Therefore, if the current limiter 6 is not provided, the current flowing to the motor 1 at the starting time becomes a value which is calculated by dividing the output voltage of the power supply 7 by the d.c. resistance and the current detecting resistor R (valued at approximately 0.1 ohm), and becomes, for example, as below. ##EQU1##
The current flowing to the motor 1 is, on the other hand, approx. 3.4 A when the motor 1 is rotated under control of the steady-state turning speed such as, for example, 3,600 r.p.m., and the inflowing current ratio of the current at the starting time to the current at the steady-state time is large. This means tht the driving circuit 2 is largely loaded, and the cost of the transistors Q.sub.1 and Q.sub.2 of the final stage of the driving circuit 2 is thus raised. To this end, the current limiter 6 has been used.
Since the prior-art spindle motor driver for the magnetic disc device is constructed as described above, the driving circuit 2 is improved by the operation of the current limiter 6, but since the limiter 6 is inserted in series between the d.c. power supply 7 and the load, i.e., between the driving circuit 2 and the motor 1, a disadvantage is that the electric power is always lost to disturb the high efficiency of driving the motor with the same power supply capacity.